1. Field of the Invention
This invention relates to electronic switching power supplies for supplying an electrical load from either an AC or a DC voltage source of an undetermined voltage level, and in the case of an AC source, of an undetermined frequency.
2. Description of the Prior Art
Electronic switching power supplies are used for supplying electrical or electronic devices with a constant voltage and/or constant current. Often such power supplies are constructed as a primary or secondary pulsed blocking oscillator, throughflow converter or push-pull converter. Generally, they are provided with a rectifier circuit connected in series with a filtering and smoothing arrangement and a transformer or choke. In cases where a transformer is utilized, the electronic switch is generally operated at a frequency of about 20 kHz, in order to keep the size of the transformer small.
It is desirable that portable electrical and electronic devices (for example, electric shavers, electronic flash units, radio and television units, etc.) can be operated at different voltages, or if these devices are provided with storage batteries, the storge batteries themselves can be charged to different voltages. Both AC and DC voltages ranging from 110 to 240 Volts and, and in the case of AC voltages, having operating frequencies between 50 Hz and 60 Hz are suitable for this purpose.
German Offenleggungschrift No. 2,751,578 discloses a switching arrangement operable to furnish a regulated voltage through the use of a blocking oscillator, having a primary coil connected with the input voltage source through a transistor which is switched in through positive feedback and through a emitter resistor. In the circuit arrangement there shown, the swith-on time of the transistor is dependent upon the input voltage, and the transistor's switch-on time decreases as the input voltage rises. As a result, the change in the switching frequency of the transistor leads to a corresponding change in the average output power. However, this known circuit arrangement is not suitable for the rapid charging of a storage battery able to supply a load independent of the line.
European Patent Application No. A1-80,107,474.1 describes a power supply which can be operated at various DC and AC voltages, as well as at various AC frequencies, without requiring a switchover. A blocking oscillator is controlled by two controllable semiconductor switches dependent upon the current flowing through the primary coil of a transformer and dependent upon the applied input voltage. The blocking oscillator is controlled such that the output power which is delivered is preferably constant. A parallel circuit consisting of a storage battery and a DC motor acts as an electrical load, such that the power supply delivers either the full motor power or, if the motor is switched off, a charge current for the storage battery. In this arrangement, the storage battery (consisting of nickel cadmium cells) is rapidly charged with a high current when the DC motor is switched off. A voltage dependent switch is provided to avoid damage to the storage battery during the rapid charging. The protective switch includes a transistor, the collector of which is connected through a resistor to the base of a switching transistor connected in series with a transformer, and the base-emitter line of which is connected in series with another resistor and a Zener diode in parallel with the storage battery. If, during charging, the storage battery exceeds a value set by the Zener diode, the transistor switches through and thereby blocks the switching transistor connected in series with the primary coil of the transformer. In this known circuit, the switched-on and switched-off times of the blocking oscillator are dependent upon currents flowing through two different resistors, such that a hysteresis behavior results from the difference in the two resistor currents. The blocking oscillator is not constantly switched back and forth between the on and off states, but the storage battery voltage must have fallen to a certain level before the charge current is again switched on. When the blocking oscillator is switched off, a light-emitting diode situated in the base circuit of the second transistor is extinguished, thus indicating a termination of the charging process.
However, such a power supply for fast-charging a storage battery with a primary pulsed blocking oscillator has a relatively low efficiency, due to the required standby voltage for supporting the control and regulation electronics. Additionally, when the supply line plug is not connected (i.e., when the device is operating solely under power supplied by the storage battery), the current is constantly being drawn from the storage battery to operate the supporting control and regulation electronics. Still further, the components required to construct such a power supply are relatively expensive.
One object of the present invention is the provision of an electronic switching power supply for supplying current to an electrical load from AC or DC voltage sources of different voltage levels, which requires little circuitry and which is suitable for the fast-charging of rechargeable batteries. It is another object of the invention that the power supply guarantee a transition from a high fast-charging current to a lower maintenance current when a set output voltage is approached or reached. Preferably, errors in detecting the battery voltage due to varying internal resistances of the battery cells are avoided, and energy is not consumed for supporting control and regulation electronics when the power supply is not connected to an AC or DC supply line.